yield*

The yield* expression is used to delegate to another generator or iterable object.

Syntax

yield* expression
expression

The expression which returns an iterable object.

Description

The yield* expression iterates over the operand and yields each value returned by it.

The value of yield* expression itself is the value returned by that iterator when it's closed (i.e., when done is true).

Examples

Delegating to another generator

In following code, values yielded by g1() are returned from next() calls just like those which are yielded by g2(). 

function* g1() {
  yield 2;
  yield 3;
  yield 4;
}

function* g2() {
  yield 1;
  yield* g1();
  yield 5;
}

const iterator = g2();

console.log(iterator.next()); // {value: 1, done: false}
console.log(iterator.next()); // {value: 2, done: false}
console.log(iterator.next()); // {value: 3, done: false}
console.log(iterator.next()); // {value: 4, done: false}
console.log(iterator.next()); // {value: 5, done: false}
console.log(iterator.next()); // {value: undefined, done: true}

Other Iterable objects

Besides generator objects, yield* can also yield other kinds of iterables (e.g., arrays, strings, or arguments objects). 

function* g3() {
  yield* [1, 2];
  yield* '34';
  yield* Array.from(arguments);
}

const iterator = g3(5, 6);

console.log(iterator.next()); // {value: 1, done: false}
console.log(iterator.next()); // {value: 2, done: false}
console.log(iterator.next()); // {value: "3", done: false}
console.log(iterator.next()); // {value: "4", done: false}
console.log(iterator.next()); // {value: 5, done: false}
console.log(iterator.next()); // {value: 6, done: false}
console.log(iterator.next()); // {value: undefined, done: true}

The value of yield* expression itself

yield* is an expression, not a statement—so it evaluates to a value.

function* g4() {
  yield* [1, 2, 3];
  return 'foo';
}

function* g5() {
  const g4ReturnValue = yield* g4();
  console.log(g4ReturnValue) // 'foo'
  return g4ReturnValue;
}

const iterator = g5();

console.log(iterator.next()); // {value: 1, done: false}
console.log(iterator.next()); // {value: 2, done: false}
console.log(iterator.next()); // {value: 3, done: false} done is false because g5 generator isn't finished, only g4
console.log(iterator.next()); // {value: 'foo', done: true}

Specifications

Specification
ECMAScript Language Specification (ECMAScript)
# sec-generator-function-definitions-runtime-semantics-evaluation

Browser compatibility

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See also